[cochino12]

Originally Posted by Jonboy

What test do they do in CO? They did a two speed idle test here, but did not put my dually on the rollers. mine passed with flying colors, but I did add some denatured alcohol to the tank before the test, just in case.

Just the tailpipe test

Originally Posted by rfmaster

From the test data, based on CO2% values I would say that your engine is running very rich at idle. From ideal Stoichiometric charts CO2=11.8% is equivalent to 13.25:1 AFR (which is very rich!) From Stoich charts running at 13.25:1 AFR -> CO=3.5%, HC=150 ppm; CO2=11.8%.

Since your engine is TBI injected it should be idling in close loop with O2 sensor keeping AFR close to 14.7:1.

It is possible that you have a contaminated O2 sensor (it will read lean mixture most of the time casing ECM to add fuel). Another possibility is exhaust leak upstream from O2 sensor - exhaust manifold to head gasket leak or between exhaust manifold and pipe (leaky donuts).

Another common problem is a weak spark or misfiring cylinder or two. Misfire results in unburnt fuel and free O2 being dumped into exhaust. O2 sensor will detect extra oxygen and ECM will add fuel which engine does not need. This results in increased HC ppm and high CO%

Another common problem that common to high millage TBI engines is a bad or flaky CTS - this sensor cost under $15, but ECM relies on it to set correct baseline mixture. Based on test data it does not appear the case since 2500 RPM data is closer to stoch ratio 14.7:1.
Sorry for the long answer.

Wow, great info. I'll start checking